Silicon carbide (SiC) has a band gap approximately three times wider, a breakdown voltage approximately ten times higher, a saturated electron drift velocity approximately twice higher, and a thermal conductivity approximately three times higher, than those of silicon (Si), and thus silicon carbide has the characteristics that are not in silicon. Further, silicon carbide is a thermally and chemically stable semiconductor material. Therefore, it is expected that a device using a silicon carbide substrate is employed as a power device that surmounts physical limitations of a device using silicon or as an environment-resistant device operating at high temperatures.
For optical devices, a material development of gallium nitride (GaN) aiming at shorter wavelengths is studied. The lattice mismatch of silicon carbide with respect to gallium nitride is significantly smaller than those of other compound semiconductors. Therefore, a silicon carbide substrate is of interest as a substrate for epitaxial growth of gallium nitride.
Such a silicon carbide substrate can be obtained by slicing, to a predetermined thickness, a single crystal silicon carbide manufactured for example by the modified Lely method. The modified Lely method is the method according to which a crucible made of graphite is provided in which a seed crystal substrate of single crystal silicon carbide is disposed in an upper portion of the crucible and silicon carbide crystal powder is contained in a lower portion thereof, an inert-gas atmosphere is provided inside the crucible, thereafter the silicon carbide crystal powder is heated to sublime the silicon carbide crystal powder, the vapor resultant from the sublimation is diff-used in the inert gas, transported to a region near the seed crystal substrate and recrystallized near the surface of the seed crystal substrate set at a low temperature, and the single crystal silicon carbide is grown on the surface of the seed crystal substrate.
The silicon carbide substrate thus obtained using the modified Lely method, however, has a problem of generating many micropipes that have openings in a surface of the silicon carbide substrate and are hollow crystal defects extending in the direction of the c-axis.
Accordingly, Japanese Patent Laying-Open No. 2004-292305 (Patent Document 1) for example discloses a method according to which a seed crystal substrate of single crystal silicon carbide and a polycrystalline silicon carbide substrate are laid on each other with a silicon source therebetween, they are contained in an airtight container, thereafter the seed crystal substrate and the polycrystalline silicon carbide substrate are heated to 1400° C. to 2300° C., the silicon source between the substrates is melted into a ultrathin silicon melt that is present between the substrates, and single crystal silicon carbide is grown on the seed crystal substrate by liquid phase epitaxial growth. The resultant micropipe density is 1/cm2 or less.
According to this method, in the heating to 1400° C. to 2300° C., the silicon melt, which enters the portion between the seed crystal substrate and the polycrystalline silicon carbide substrate located on the seed crystal substrate, forms a silicon melt layer of approximately 30 μm to 50 μm in thickness at the interface between these substrates. The silicon melt layer becomes thinner as the heating temperature rises to finally become approximately 30 μm in thickness. Then, carbon atoms flowing out from the polycrystalline silicon carbide substrate are supplied through the silicon melt layer onto the seed crystal substrate, and the single crystal silicon carbide is grown on the seed crystal substrate by liquid phase epitaxial growth. It is disclosed that, in a surface of the liquid-phase epitaxially grown single crystal silicon carbide, the micropipe defect density is 1/cm2 or less.
Patent Document 1: Japanese Patent Laying-Open No. 2004-292305